Gently inspect your squash blossoms for any bugs or debris. Use a damp paper towel to clean them, if needed. Remove the stamens or pistils inside, then pat dry.
In a small bowl, mix together Greek yogurt, garlic, lemon zest and juice, fresh herbs, nutritional yeast, salt, and pepper. Taste and adjust seasoning — it should be bright and tangy, but not watery.
Transfer the filling to a piping bag or a zip-top bag with the corner snipped off. Gently pipe a small amount of filling into each blossom, then twist the tops to close. Keep them chilled while you heat your oil.
Fill a medium saucepan or deep fryer with about 2 inches of oil. Heat to 350°F (175°C). Use a thermometer if you have one for best results.
In a medium bowl, whisk together the flour, cornstarch, and a pinch of salt. Add the ice-cold sparkling water and stir gently — it should be slightly lumpy. Do not overmix.
Lightly dust each stuffed blossom with a little flour to help the batter stick. Dip into the tempura batter, letting excess drip off, then carefully place into the hot oil. Fry 2–3 at a time to avoid crowding.
Fry each blossom for about 1–2 minutes per side, or until lightly golden and crisp. Remove with a slotted spoon and drain on paper towels.
While still hot, sprinkle with flaky sea salt. Serve immediately and enjoy the delicate crunch and creamy, herby filling!